fix(studio): theme picker confirms applied + surfaces load failures

Two operator-visible fixes prompted by 'I picked a theme but
nothing changed':

  - Theme picker tile taps now show a brief "Theme applied:
    <name>" snackbar — the picker applies themes instantly, so
    the operator needs a confirming cue. Slow `_pluginThemes`
    loads no longer read as "click did nothing".
  - `_pluginThemes` failures (plugin unreachable, manifest
    drift, etc.) write to `FaiLog` so `fai admin doctor` and
    Studio's inline log viewer can surface them. Previously
    swallowed silently, which is what made debugging this so
    miserable.
  - Settings dialog's primary button relabelled to
    "Connect to endpoint" (was "Save & connect") so operators
    don't mistake it for "save my theme choice". The theme
    selection persists at tile-tap time; the Settings dialog
    has nothing left to "save".
  - Theme section header gains an explicit "(applies instantly)"
    cue for the same reason.

Also fixes a pre-existing curly-braces lint in the n8n add-
endpoint dialog. Studio bumped to 0.62.1, editor pinned via
path override at 0.15.1.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/l10n/app_localizations_en.dart

@@ -313,7 +313,16 @@ class AppLocalizationsEn extends AppLocalizations {
       'Server text in English — Studio shows it as supplied.';
 
   @override
-  String get themePluginHeader => 'Theme plugin';
+  String get themePluginHeader => 'Theme plugin (applies instantly)';
+
+  @override
+  String themeApplied(String name) {
+    return 'Theme applied: $name';
+  }
+
+  @override
+  String get themePluginFailed =>
+      'Theme plugin could not load — keeping previous theme.';
 
   @override
   String get themePluginNone => 'Built-in (no plugin)';
@@ -358,7 +367,7 @@ class AppLocalizationsEn extends AppLocalizations {
   String get settingsTls => 'TLS';
 
   @override
-  String get settingsSaveAndConnect => 'Save & connect';
+  String get settingsSaveAndConnect => 'Connect to endpoint';
 
   @override
   String get settingsLanguage => 'Language';